Lithographic techniques using light (photolithography) or electron beams (electron beam lithography) have been known for the fabrication of fine structures on the order of several tens of nanometers to several hundreds of nanometers, and a variety of semiconductor devices are fabricated using these techniques.
The former photolithography includes complicated processes of irradiating a surface of a resin film with a reduced pattern of UV (ultraviolet) light corresponding to interconnections, and developing a latent image of the irradiated pattern. The photolithography also has a theoretical or fundamental lower limit of processable dimensions, because there occurs light diffraction, and photolithographic processing with dimensions of 100 nm or less is substantially difficult. On the other hand, the latter electron beam lithography enables processing with dimensions smaller than those in photolithography using UV (ultraviolet) light. However, it takes much time to pattern a large number of substrates, because patterning (imaging) is conducted directly with electron beams. For these reasons, it is difficult to obtain high throughputs according to these known lithographic techniques.
In contrast, a technique of fabricating a fine structure, called “nanoimprinting”, has been reported in a variety of documents as a technique yielding a high throughput. According to this technique, a desired convex-concave pattern is formed typically on a silicon substrate or a metal sheet, and the silicon substrate or metal sheet bearing the pattern is pressed to a resinous film which is generally heated to a temperature higher than its glass transition point to transfer the convex-concave pattern of the original to the resinous film to thereby form a corresponding convex-concave pattern, which is reversal to the original pattern, on the resinous film.
Japanese Unexamined Patent Application Publication (JP-A) No. 2004-211021 discloses use of an epoxy resin-precursor composition containing a fluorine-containing compound as a monomeric precursor for epoxy resin, to obtain an optical device excellent in dimensional and optical properties. Japanese Unexamined Patent Application Publication (JP-A) No. 2005-074774 discloses that, to finely process a mold, the mold is subjected to dry etching with fluorine gas to fix fluorine atom on the surface of the mold to thereby impart mold releasability to the mold.
Materials of resinous films for these purposes include, for example, thermoplastic resins such as a poly(methyl methacrylate) (PMMA) and a polystyrene; crosslinked polymers thereof; and thermosetting resins such as a polyimide. Such nanoimprinting yields, for example, patterns having a structure composed of resinous pillars with diameters of several tens of nanometers to several hundreds of nanometers arranged on a substrate, or a structure composed of bumps or grooves.
However, dimensions to be processed are decreased more and more, and, when a polymeric resist film is subjected to processing to form pillars with diameters of several tens of nanometers thereon, the mold should be pressed to the polymeric resist film under a higher pressure, and thereby the surface of mold and a target fine resist pattern structure once formed are likely to be destroyed. In addition, the known nanoimprinting technique should use a resist that is susceptible to deformation, but this causes lack of rigidity of the formed pillars, and such pillars may often fail to maintain their fine dimensions.
In contrast to the nanoimprinting technique using a polymeric film, there is known another imprinting technique using a photopolymerizable monomeric resist-precursor composition. The term “precursor” means that it exhibits an activity as a resist only after photocure. The precursor herein is a composition composed of one or more different photopolymerizable monomers or oligomers.
This process is called photonanoimprinting or step-and-flash imprint lithography. In this process, for example, a liquid resist precursor is added dropwise onto a substrate; a mold is pressed thereto so as to allow the liquid resist precursor to follow a surface convex-concave pattern of the mold; and UV (ultraviolet) light is applied to the resist precursor to photocure the precursor. This process is advantageous in that the pattern on the mold can be easily transferred even when the mold is pressed under a relatively low pressure, because the resist is in the form of a liquid precursor. The photonanoimprinting is further advantageous in that there is no need of using a precise optical system for the application of UV (ultraviolet) light, and an entire patterning process can be constructed at low cost.
In addition, the technique is remarkably advantageous in that dimensions to be processed are resistant to variation depending on the wavelength of UV (ultraviolet) light to be applied, because the shape and dimensions of the resist to be processed are determined by the fine convex-concave pattern previously arranged on the mold. Therefore, the photonanoimprinting is often used for patterning relatively fine structures. When a quartz plate, which is transparent to UV (ultraviolet) light within abroad range of wavelengths, is used as the substrate, the resist precursor can be exposed to UV (ultraviolet) light through the quartz mold. Likewise, when a substrate used is transparent to UV (ultraviolet) light, the resist precursor may be exposed to UV (ultraviolet) light through the substrate.
First of all, the resist precursor for this purpose should be a liquid, as mentioned above. Such a photocurable resist precursor used in photonanoimprint has to be easily spread as a result of pressing of the mold. Accordingly, the resist precursor as a liquid is preferably applied as a liquid film typically by spin coating. The thickness of the liquid film is determined depending on dimensions to be processed, and it is generally from 50 nm to 100 nm in many cases.